A conventional closed-circuit video surveillance system, indicated generally by reference numeral 10, is illustrated in block diagram form in FIG. 1.
The surveillance system 10 includes a host computer 12, which controls the operation of the system, a video switch 14 which selectively interconnects input video signals (represented by dashed lines 16) with outputs of the switch (represented by dashed lines 18). Also included in the system 10 are video cameras 20, monitors 22, control consoles 24, and sensor devices 26. A local area network 28 is provided for interchange of command, status and alarm data between the system host 12 and the cameras 20, control consoles 24 and sensors 26.
A user interface 30 is also provided to permit a system operator to control operations of the system. The interface 30 may incorporate a monitor and a control console which are not shown separately from the user interface 30. Also included in the system 10 are one or more video cassette recorders 32 which are available to record video signals that are generated by the video cameras 20 and selectively routed to the VCRs 32 by way of the video switch 14. Video signals previously recorded by the VCRs 32 can be played back by the VCRs 32 and selectively routed through the switch 14 for viewing on one or more of the monitors 22. As is well known to those who are skilled in the art, system host 12 also controls the video switch 14 so that, in response to control signals entered via the control consoles 24, video signals generated by a selected one of the video cameras 20 is routed for viewing at a selected one of the monitors 22. Sensors 26 generate alarm signals in response to anomalous events such as unauthorized opening of doors, windows or display cases. The system host 12 operates to inform the human operator of the alarm conditions, and predetermined camera movements and routing of corresponding video signals to pre-selected monitors 22 may be automatically actuated in response to alarm signals. Movement of the cameras can also be controlled through commands issued by the system host, in response to pre-programmed surveillance routines, or in response to operator input through the control consoles 24 or the user interface 30.
It is generally a desired goal in video surveillance systems that video signals representative of conditions before, during and after the onset of an alarm event be recorded for current and future reference, subsequent legal proceedings, and so forth, and it is for this purpose that the VCRs 32 are provided. However, surveillance systems frequently include a large number of video cameras (e.g., dozens or hundreds of cameras), and the cost of video cassette recorders is too high to permit recording of the signals generated by more than a few of the cameras at any given time. In conventional systems, video signals indicative of conditions prior to an alarm event cannot be recorded, unless the output of a video camera has been routed for recording by a VCR prior to the time an alarm event occurs.
To maximize the recording capacity of the VCRs, it is known to provide space-division-multiplexed (e.g., quad multiplexed) signals for recording on VCRs 32, and it is also known to operate the VCRs in a time lapse mode, with a reduced frame rate and switching among a number of cameras. When an alarm occurs, the time lapse mode can be automatically stopped in favor of a standard frame rate recording operation for the critical camera or cameras. In addition, critical cameras can be connected at all times for recording by a dedicated VCR. There are, however, disadvantages with all of these techniques. As noted above, it is quite expensive to provide a large number of VCRs, and if space-division-multiplexing is employed, the spatial resolution of the image signal is reduced, which may impair recognition of critical elements of the recorded images. On the other hand, time lapse recording suffers from reduced temporal resolution, so that again critical events may be missed. And, as noted before, if a camera having a view of an alarm event is not being recorded prior to the alarm event, the critical video information may simply fail to be captured.
An "intrusion monitoring system" disclosed in U.S. Pat. Nos. 5,109,278 and 5,111,291 (assigned to Commonwealth Edison) attempts to address some of these problems by providing an analog-to-digital converter and frame buffers at the output of a video multiplexer. Although the system disclosed in the Com Ed patents provides some capability for capturing and storing signals indicative of conditions at or before the time that an alarm event occurs, the system does not seem to be adapted to operate with a large number of cameras, and the temporal resolution is poor, especially when the system includes more than a minimal number of cameras.